1. Field of the Invention
The present invention relates to a realtime sensitivity correction method for infrared sensors and an infrared imaging system employing a realtime sensitivity correction mechanism. More particularly, the present invention relates to a sensitivity correction method which compensates for the variations in sensitivity levels among a plurality of elements that constitute an infrared sensor device, and to an infrared imaging system having a function to correct the sensitivity variations among the infrared sensor elements.
2. Description of the Related Art
Infrared imaging systems are instruments that capture infrared images, or thermographs, of a target object by sensing infrared rays radiated from its surfaces. They are utilized in various industrial fields, for example, to observe temperature distribution on target surfaces or to detect the shape of an object. Infrared imaging systems fall roughly into two categories according to the wavelengths of infrared rays that they sense; one is for 3-5 .mu.m band, and the other is for 8-10 .mu.m band. The 8-10 .mu.m band systems mainly use HgCdTe sensors, while the 3-5 .mu.m band systems use infrared sensors made of PtSi, InSn, HgCdTe, or the like. With their material composition varied, HgCdTe sensors can be applied to relatively wide wavelength ranges.
Regarding the structure of sensor devices, one-dimensional (or linear) arrays are typically used for the above purposes, while there are other types of infrared sensors, such as ones composed of discrete elements or two-dimensional arrays. One-dimensional and two-dimensional arrays contain a plurality of sensor elements, and ideally, it is desirable that all elements provide uniform responses. In reality, however, some variations in sensitivity levels inevitably exist among the elements constituting an infrared sensor. This sensitivity variation will result in non-uniform sensor outputs for a target surface having a flat temperature distribution, thus giving inaccurate target images. To improve this situation, conventional infrared imaging systems employ a sensitivity correction mechanism that compensates for the unevenness of individual sensor elements by applying appropriate data processing to the detected signals.
This kind of sensitivity correction mechanisms would properly work as long as the sensor elements keep their initial characteristics. However, since the individual elements actually vary with time, it is hard for the above correction mechanisms to maintain their long-term accuracy of sensitivity compensation.
To solve this problem, another sensitivity correction method is proposed. This method uses reference heat sources being controlled at constant temperatures. Scanning the reference heat sources, the infrared sensor outputs reference temperature detection data. This data is used to quantify the sensitivity variations among a plurality of sensor elements, allowing compensation for them to be conducted in a later stage.
FIG. 28 shows a typical conventional infrared imaging system. Infrared rays emanating from object surfaces first enter an optical system 301, then pass through another optical system 302 having a scanning capability, and finally reach a linear infrared sensor 303. The analog detection signal produced by the infrared sensor 303 is amplified by an amplifier 304 and fed to an analog-to-digital (A/D) converter 305. The resultant digital detection signal is then subjected to a signal processing circuit 306 for sensitivity correction and other necessary processes. After that, a digital-to-analog (D/A) converter 307 converts the corrected signal back to an analog signal, thus allowing the captured and corrected infrared image to be displayed on a video monitor 308.
This infrared imaging system employs two reference heat sources 310 and 311 to compensate for the sensitivity differences among sensor elements as described earlier. The reference heat sources 310 and 311 are regulated to keep their respective temperatures. Their infrared outputs are given to the infrared sensor 303 by an optical system 302 during a part of the system's scanning cycle. More specifically, the system scans the target object at regular intervals. Each scanning cycle consists of an "effective scanning period" and a "non-effective scanning period." During the effective scanning period, the optical system 302 actually scans the target surfaces. Using the remaining time, or the non-effective scanning period, it scans the reference heat sources 310 and 311, thus enabling the infrared sensor 303 to output detection signals for the two different reference temperatures. The signal processing circuit 306 then processes these detection signals to calculate parameters to compensate for the sensitivity variations among the sensor elements.
FIG. 29 is a diagram which shows the structure of the optical systems of FIG. 28. To form an image of the target, the first optical system 301 comprises lenses 312 and 313, and the second optical system 302 comprises lenses 314 and 315. The second optical system further comprises two more lenses 316 and 316 to collect infrared rays emanating from the reference heat sources 310 and 311, together with two reflectors 317 and 319 to direct the rays to the infrared sensor 303. The linear infrared sensor 303 is disposed at the back of this optical system 302 in such a way that the array will be orthogonal to the direction of optical scanning. That is, the infrared imaging system scans the target both electronically (by the linear infrared sensor 303 itself) and optically (by the optical system 302), thus achieving a two-dimensional scanning operation.
Typically, the above-described scanning operation of the optical system 302 is conducted in concert with the raster scanning operation of the video monitor 308. In the case of interlaced video, for example, one complete picture, or frame, is obtained as a combination of two separate field scans. Here, the term "field" refers to a set of alternating lines in an interlaced video frame. In synchronization with the video monitor 308, the optical system 302 scans odd-numbered lines in one field and then even-numbered lines in the next field. The signal processing circuit 306 joins the infrared detection signals obtained in those two scans, thereby constructing a complete infrared image of the target.
During the non-effective scanning period, the optical system 302 scans the reference heat sources 310 and 311 being regulated at constant temperatures, thus directing their infrared rays to the infrared sensor 303. The unevenness in the infrared sensor outputs is measured in one non-effective scanning period, and this measurement result is used in the next or later effective scanning period(s) to correct the sensitivity of each individual sensor element.
The reference heat sources 310 and 311 can be implemented with Peltier effect devices, for example, with supply currents being controlled so that they will keep their respective set temperatures. While FIG. 29 illustrates a specific arrangement where the two reference heat sources 310 and 311 are placed separately across the center line, it is also possible to place both on one side. These two reference heat sources 310 and 311 provide a high and low temperatures determined according to the range of target temperatures that the infrared sensor 303 can detect. They are controlled so that they keep a predetermined temperature difference. This means that the system can make a sensitivity correction at least at two points within the detection temperature range of sensor elements.
The video monitor 308 typically uses a cathode ray tube (CRT) to display the captured infrared images. Video signals entered to the video monitor 308 conform to the National Television System Committee (NTSC) standard, which defines an interlaced video format where each frame is composed of two fields. In the NTSC format, one frame scanning cycle is 1/30 of a second, while one field scanning cycle is 1/60 of a second.
As mentioned earlier, the sensitivity variations among infrared sensor elements will degrade the accuracy of infrared images appearing on the video monitor 308. In other words, the target temperature distribution displayed on the video monitor 308 can be distorted because of the unevenness of the sensor's sensitivity levels. Particularly when the target temperature distribution dynamically changes with time at a relatively high rate, the above inaccuracies of displayed infrared images would make it difficult for the user to view the temperature changes.
As long as the target is stationary and its surface temperature distribution is time-invariant, conventional infrared imaging systems will provide near satisfactory results in terms of the accuracy of displayed infrared images, although they perform sensitivity correction only in an intermittent manner. However, when the target temperature distribution changes with time at a relatively high rate, the conventional correction mechanisms cannot catch up with the changes, thus causing inaccurate images to be displayed.
In addition to the sensitivity variations discussed above, there is another issue that should be considered when using one-dimensional or two-dimensional infrared sensors. These sensors contain a plurality of sensor elements, but some of them may exhibit abnormal response, such as too low output levels or too much noises, which are regarded as partial defects. The problem is that such faulty elements cannot be corrected by the aforementioned sensitivity correction mechanism, and infrared images of an object with a uniform temperature distribution would exhibit some visible errors. To address this problem, a pixel interpolation technique is implemented in conventional infrared imaging systems employ, as part of their image data processing capabilities. That is, the pixel data for a faulty sensor element is imitated by interpolating the outputs of neighboring normal elements.
In conventional infrared imaging systems, however, the presence of faulty sensor elements can be detected at their power-up time or in response to an operator's instruction. While defects can happen at any time during the operation, the conventional systems are unable to immediately find such a defect of a sensor element and to cope with the defect in a prompt manner. Since no compensation is effected, infrared images displayed on a monitor screen in this situation could not be satisfactory ones.